Low-power and lossy networks (LLNs) are important in a wide range of communication applications, such as advanced metering infrastructure (AMI), industrial automation, building and some automation, and environment monitoring. Several Internet Engineering Task Force (IETF) Working Groups (WGs) have been working on IPv6 based standards for LLNs. The 6LoWPAN (IPv6 over Low-power Wireless Personal Area Networks) WG has published six standards, including neighboring node discovery and header compression for LLNs. The ROLL (Routing Over LLNs) WG has completed eleven standards including IPv6 Routing Protocol for LLNs (RPL) and routing metrics used for path determination in LLNs. The CoRE (Constrained RESTful Environment) WG is publishing Constrained Application Protocol (CoAP). Many other standards are under development. ZigBee Alliance and IPSO (Internet Protocol for Smart Objects) Alliance are implementing protocol stack and performing interoperability testing. Industrial deployments of LLNs are also in progress.
In LLNs, nodes and communication links are severely constrained. Nodes operate with resource constrains on processing power, memory, power consumption, and the like. TWO classes of constrained nodes are defined. A class 1 node has about 10 kB (kilobytes) RAM and 100 kB ROM, and a class 2 node has about 50 kB RAM and 250 kB ROM. The communication links between the nodes can be characterized by a high loss rate, low data rate, instability, low transmission power, and short transmission range. There can be from a few dozen up to millions of nodes within a practical LLN. Nodes in LLNs are typically deployed in extended outdoor and indoor environments. Due to the extremely constrained resources and large scale deployment. LLN nodes typically form wireless mesh network and communicate in a multi-hop manner.
In contrast with other networks, the LLNs can also have constrained traffic pattern. Multipoint-to-point, e.g., from nodes inside the LLN towards a central control or data concentrator node, traffic is dominant. The point-to-multipoint, e.g., from a central control node to a subset of nodes inside the LLN, traffic is less common. The point-to-point, e.g., between nodes in the LLN, traffic is rare. Control node in the LLN usually acts as data sink and collects data from all other nodes in LLNs.
Data collection in LLNs such as smart meter networks is required to have high reliability. However, LLN by itself is a constrained node network. This presents challenges to routing procedures. Conventional peer-to-peer routing protocols, such as AODV (Ad-hoc On-demand Distance Vector) and DSR (Dynamic Source Routing) are not suitable for routing in LLNs. As a result, the IETF ROLL WG has developed RPL as a routing protocol to accomplish multipoint-to-point traffic dominant routing in LLNs.
The performance of RPL has been evaluated and analyzed in “Performance Analysis of the RPL Routing Protocol” by Accettura et al, IEEE ICM, 2011. They conclude that RPL exhibits fast network setup. In “The Role of the RPL Routing Protocol for Smart Grid Communications,” IEEE Communications Magazine, Vol. 51 No. 1, 2013), Ancillotti et al. state that RPL shows good scalability, but may suffer from severe unreliability due to suboptimal route selection. In “Performance and Route Stability Analysis of RPL Protocol,” KTH Royal Institute of Technology, 2012, Khan demonstrates that frequent route change has negative impact on the performance, of RPL.
U.S. Publication 20120099587 describes an opportunistic method to improve RPL reliability. A node transmits a data packet to multiple parent nodes of the same rank and the parent performs channel access to determine if the data packet has been forwarded by parents of siblings. If not, the parent forwards the data packet towards the sink. If yes, the parent drops a local copy of the data packet. However, issues with this method include redundancy and communication overhead. If two sibling parents are not within radio range, at least two copies of same data packet are forwarded to the sink. Transmission of same packet along multiple paths increases communication overhead and the probability of interference with other transmission.
The reliability of the RPL can be improved by selecting optimal routes. To select optimal routes, the key is to select proper routing metrics, e.g., a hop count. However, a short route may be composed of low quality links and therefore, does not necessarily provide high reliability. The IETF standard RFC 6551 defines routing metrics for RPL including hop count, link quality, etc. Other metrics are also proposed for RPL. For example, In “Wireless Mesh Network Communication Unit for Smart Meters Enabling Low-Cost and Stable Communication in Smart Grid Systems,” Toshiba Corporation, 2012, Yoneyama et al. describe using the received signal strength and retransmission rate as the metrics for RPL to achieve low cost and stable communication in smart grid systems. In “Load Balanced. Routing for Low Power and Lossy Networks,” IEEE WCNC, 2013, Liu et al. describe a workload based metric to improve reliability of RPL. However, a stability based metric is still missing.
Discovering stable routes is very important in LLNs because stable routes persist for a long time. Short term routes in the wireless networks make the network topology unstable. Unstable routes can be caused by unpredictable occurrences of obstacles, unpredictable signal interference, etc.
Stability based routing metric provides numerous benefits to LLNs, such as improving network reliability, reducing communication overhead, mitigating interference, increasing bandwidth utilization, increasing energy efficiency, and prolonging network operating duration, especially for battery-powered networks. With stable routes, nodes do not need to make frequent route changing and therefore, transmit fewer control messages that may interfere with data packet transmission. As a result, nodes use more time and more bandwidth for data packet transmission, which, in turn, increases packet delivery rate. Accordingly, it is desirable to provide stability based routing metric not only for RPL routing protocol but for all routing procedures in wireless networks.